1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) and a manufacturing method for the same, and particularly to an active matrix type LCD using thin film transistors (TFTs) as pixel switching elements and a manufacturing method for the same.
2. Description of Related Art
In recent years, in a field of display devices using semiconductor devices, LCDs characterized by reduced energy and space are rapidly becoming common in place of CRTs. An LCD has a plurality of electrodes, lines and devices over a transparent insulating substrate. To be more specific, active matrix type LCDs are widely used in which switching devices such as thin film transistors (TFTs) having scanning and signal lines, gate, source and drain electrodes are provided in array and an independent video signal is applied to electrodes of each display pixel.
To manufacture this active matrix type LCD, a manufacturing method that performs five photolithography processes (hereinafter referred to as a five mask process) has been common as disclosed in Japanese Unexamined Patent Application Publication No. 10-268353.
In the five mask process, wet etching by wet chemical etchant or dry etching by reactive gases can be used in patterning source electrodes and lines of TFTs. Although wet etching is inferior in miniaturization and processing accuracy to dry etching, wet etching is superior in costs such as equipment prices and running costs to dry etching.
In wet etching, if there is even a slight gap between a metal film which is etched to form source electrodes and lines and its lower layer, the etchant may penetrate into the gap and which may cause source electrodes and lines to disconnect. This disconnection tends to occur if the lower layer of the metal film for source electrodes and lines has a bump. Further, if the lower layer is composed of semiconductor such as amorphous silicon, photoresist residue of the previous process tends to remain on the lower layer. The photoresist residue may also cause disconnection. Therefore, the disconnection may most easily occur if the lower layer composed of semiconductor has a bump.
To prevent disconnection, an LCD shown in FIG. 8 is disclosed in Japanese Unexamined Patent Application Publication No. 2001-125140. The LCD shown in FIG. 8 has a gate electrode and line 2, a source line 6a, a source electrode 6b, a drain electrode 7 and a semiconductor layer 10. The semiconductor layer 10 is island-shaped under the source line 6a which covers the whole semiconductor layer 10. Thus, etchant penetration can be prevented. On the other hand, an island-shaped semiconductor layer is also disclosed to prevent photocurrent in Japanese Unexamined Patent Application Publications No. 2003-303973 and No. 2-830, for example.
By the way, high aperture ratio is required to match high-resolution LCDs in recent years. Thus, although finer source lines are requested, there is a limit to thinning of source lines of the LCD shown in Japanese Unexamined Patent Application Publication No. 2001-125140.